The present invention is directed to a circuit arrangement having a pre-amplifier connected to an optoelectric transducer whose gain is controllable by a final control element formed by a field effect transistor.
Such a circuit arrangement is disclosed by German Pat. No. 32 04 839.
At its input, the prior art circuit arrangement has a photodiode and contains an iterative circuit composed of an emitter stage and a collector stage. A resistor in a feedback branch is arranged between the output of the collector stage and the input of the emitter stage. This circuit arrangement composed of two stages forms what is referred as a transimpedance amplifier that has a relatively low input resistance and an extremely low output resistance. The drain-source path of a field effect transistor whose resistance is continuously controllable by a control voltage is in parallel to the collector resistor of the emitter stage. The effective collector resistance can be varied within broad limits in this manner. As a result an external setting of the gain of the first amplifier stage within a relatively great dynamic range is possible.
The prior art circuit arrangement should also exhibit the usual requirements of optimum signal to noise ratio of the electrical signals emitted by the photoreceiver, high gain and low power consumption together with an optimally low set noise.